Method of fabricating cathode ray tube screen



NOW 9 1966 A. M. MORRELL; EBAL 3,232,591

METHOD OF FABRICATING CATHODE RAY TUBE SCREEN 4 Sheets-Sheet 1 Filed July 6. 1962 A K mm J 6 O f I 4 3. T H W. W? m M w i 0 3 a m 9/ i a N 5 a a 0. Wm f L MH W W W mw m m a fly v y I, w 0/, mm f Mm m fi MM m WM W a 95 z W a 6 M m 45 j m r W 9 w we w; M v swx M55 a P 5 we? d a Nov. 1, 1966 Filed July 6, 1962 A. M. MORRELL. ET AL 3,282,891

METHOD OF FABRICATING CATHODE RAY TUBE SCREEN 4 Sheets-Sheet 2 BY 8 1755 9f 600/65) 1966 A. M. MORRELL ETAL 3,232,691

METHOD OF FABRICATING CATHODE RAY TUBE SCREEN Filed July 6, 1962 4 Sheets-Sheet 5 I I I I I IN NTORI flay/M fiamm BY Hrs 42.0% Gap/my if! d METHOD OF FABRICATING CATHODE HAY TUBE SCREEN Filed July 6, 1962 Nov. 1, 1966 A. M. MORRELL ETAL 4 Sheets-Sheet 4 MN mq R %N A m K F /3\\\\ s Q wk INVENTOR! Q \Q m R F N I l I United States Patent Oflice 3,282,691 Patented Nov. 1, 1966 3,282,691 METHOD OF FABRICATING CATHODE RAY TUBE SCREEN Albert M. Morrell, East Petershurg, and Richard H. Godfrey, Lancaster, Pa, assignors to Radio Corporation of America, a corporation of Delaware Filed July 6, 1962, Ser. No. 208,044 13 Claims. (Cl. 9635) This invention relates to color cathode ray tubes of the type comprising a plurality of electric guns, an apertured shadow mask, and a mosaic screen of systematically arrayed phosphor dots. The invention is particularly related to such tubes having an array of three guns, to improved mosaic screens for such tubes, and to a method of and apparatus for fabricating such screens.

The phosphor dots of the screen of such a tube may be laid down in trios (groups of three dots of different phosphors) by a known direct photographic printing technique, wherein a photosensitive coating on the faceplate of the tube isexposed to a point source of light through the apertures of the shadow mask. Phosphor powder may, e.g., be mixed directly with the photosensitive coating before application to the faceplate or else applied to the photosensitive coating after it has been exposed.

In operation of the tube, the electron beams are subjected to scanning forces and dynamic convergence forces, both of which affect the electron beam paths (and hence the landing spots of the electron beams on the screen) in a way that the screen-printing light rays are not affected. Unless some degree of compensation is made for the differences between the electron beam and light ray paths, serious misregister of the beam spots with the phosphor dots will result, that is, the spot and dot centers will not coincide.

Hereinafter the phosphor dots may be referred to simply as dots as distinguished from the beam spots which may be referred to simply as spots.

Misregister of the type wherein a trio of beam spots is shifted as a unit radially outward away from its associated trio of phosphor dots because of an axial shift of the deflection centers of the electron beams with changing angles of deflection, is termed radial misregister.

Misregister of the type wherein the individual beam spots of a spot trio are all three moved away from each other principally because of a deflection of the beams to obtain dynamic convergence, is termed degrouping misregister.

Misregister of the type wherein beam spots of a trio are distorted away from equilateralism because of nonuniformities of the raster scanning deflection fields and/or the phosphor dots of a trio are distorted away from equilateralism because of inherent characteristics of the dot printing system, is herein termed astigmatic misregister.

Misregister of the type wherein a phosphor dot and its associated beam spot are individually moved different distances relative to the centers of their respective trios due to an apparent decrease of the spacing of the dot printing light sources and of the electron beams from the central tube axis at increasing angles of deflection, is termed foreshortening misregister.

All four of the above-mentioned types of misregister and their causes are hereinafter more fully described.

In the commercial manufacture of prior art tubes of the type in question, compensation for radial, degrouping and foreshortening misregisters has, to a degree, been provided in the form of selected tube geometry and in the use of unique screen fabrication methods and apparatus. One example of such compensation is taught in US. Patent 2,885,935, issued to D. W. Epstein et al. In the case of tubes having a maximum beam deflection angle of 70, the Epstein et a1. technique has proved to be entirely satisfactory for home television. Yet in tubes with larger deflection angles, e.g., 90, the diflferences between the electron beam paths and the screenprinting light ray paths are great enough so that improvement over prior art techniques for providing this compensation are desirable, even for home television purposes.

Spot-dot misregister in prior art tubes occurs primarily because of an inherent characteristic of prior art screen fabrication techniques, viz., as the dot trios near the edge of the screen (where the greatest degrouping correction is needed) are enlarged through optically directing the dot-printing light rays so that these dot trios correspond in size with their associated beam spot trios, the shapes of these dot trios are undesirably astigmatically distorted to a shape other than that of their associated spot trios. Thus, obtaining of the desired dot trio size conflicts with the obtaining of the desired dot trio shape. According to prior art design parameters, average dot trio size at the edge of the screen is made smaller than the dot trio size at the center of the screen in order to limit the undesirable dot trio shape distortion. At the same time, in order to obtain an acceptable dot trio size at the edge of the screen, some degree of astigmatic distortion of the dot trios at the edge of the screen is accepted.

As used herein, the size of a phosphor dot trio or a beam spot trio is defined as being proportional to the average of the distances between the center of the trio and each of the centers of the dots or spots thereof. The center of the trio is the center of a circle inscribed in the triangle whose vertexes lie at the centers of the dots or spots of the trio.

An object of this invention is to provide a new and improved method of fabricating a mosaic dot screen for a plural beam color cathode ray tube which exhibits reduced misregister of the beam spots with the phosphor dots.

A cathode ray tube of the type described includes an improved mosaic dot screen in which the size of dot trios adjacent to the edge thereof is larger than the size of the dot trios at the center of the screen and more closely approaches the size of the associated beam spot trios than do the dot trios in prior art tubes of this type. The shapes of the phosphor dot trios are similar to the shapes of astigmatically distorted beam spot trios which result when a conventional magnetic deflection yoke is used to scan the beams over the screen and dynamic convergence is simultaneously applied thereto. Both the increased size of the dot trios at the edge of the screen and the novel shaping of the dot trios serve to more nearly concentrically register the beam spots with the phosphor dots over the entire viewing area of the screen.

According to one feature of the invention, the abovedescribed improved mosaic screen is obtained by a unique positioning of the dot-printing light source. This positioning is such as to be characterized by one or more of the following conditions: (a) the light sources are located at points such that the light rays which print a given hosphor dot pass through a different mask aperture than does an electron beam when it excites that given dot; (b) the light sources are located at points such that the paths of light rays which print the dots of the same dot trio are convergent in the region between the mask and screen.

According to another feature of the invention, foreshortening misregister is prevented, and astigmatic mis 3 register is at the same time further reduced, by disposing in the paths of the dot-printing light rays suitable light refractive means which will effect a desired differential amount of shift of the phosphor dots toward the centers of their trios.

The invention contemplates that radial spot-dot misregister correction preferably be provided using a modification of prior art light refractive devices such as those taught, e.g., in U.S. Patents 2,817,276 issued on December 24, 1957, or 2,885,935, issued on May 12, 1959, both to Epstein et al. The invention further contemplates that the different light refractive properties desired for correcting astigmatic, radial, and foreshortening misregister errors may be provided either in a plurality of refractive elements, or that the desired'refractive properties may be integrated and provided in a single refractive element.

In the drawings:

FIG. 1 is a schematic illustration of a three-beam shadow mask cathode ray tube showing the causes of radial and degrouping spot-dot misregister;

FIG. 2 is a diagram showing the relationship between a system of second order color centers and their associated first order color center;

FIG. 3 is a diagram comparing the effects of phosphor dot printing from first order color centers with the effects of phosphor dot printing from second order color centers;

FIGS. 4, 5, and 6 are schematic illustrations showing the relationship of a phosphor dot trio and an associated beam spot trio at various screen positions of prior art tubes;

FIGS. 7, 8 and 9 are schematic illustrations showing the relationship of a phosphor dot trio and an associated beam spot trio at various screen positions of a cathode ray tube made according to this invention;

FIG. 10 is a partial section of apparatus suitable for practicing the method of the invention; and

FIGS. 11, 12, and 13 are diagrams useful in explaining a procedure for practicing the invention.

FIG. 1 illustrates a cathode ray tube of the type described which comprises an envelope 10 containing therein a plurality of electron guns 11, 12, and 13, dynamic beam convergence means 14, an apertured shadow mask 15, and a dot mosaic phosphor screen 16. A magnetic beam deflection means, e.g., a magnetic deflection yoke 17 is disposed around the neck of the envelope 10 for scanning the electron beams of the guns 11, 12, and 13 in mutually perpendicular directions over the shadow mask 15 and phosphor screen 16.

The electron guns 11, 12, and 13 may, for example, be disposed co-planar or in a trangular array. A delta triangular array symmetrically disposed about the central axis AA of the tube is preferred.

If a delta array of three guns is used, the shadow mask 15 is provided with a multiplicity of apertures systematically hexagonally arrayed thereover; and the mosaic screen 16 includes a multiplicity of phosphor dots also systematically hexagonally arrayed, with a trio of three dots, each of a different color emitting phosphor, being provided for each of the apertures in the shadow mask 15. If a co-planar or non equilateral trangular gun array is used an aperture array in the mask is provided which operatively corresponds thereto.

In operation of the tube, three separate electron beams are projected from the guns 11, 12, and 13 and are directed to be converged to a cross-over point at the screen 16 by virtue of the mechanical disposition of the guns and/or convergence forces generated by the convergence means 14. The beams impinge upon the mask 15 and portions thereof pass through the mask apertures. Because of their angular relationship the portions of three beams passing through any given mask aperture are divergent in their transit from the mask to the screen and thus excite different color-emitting phosphor dots of the same 4 invention, in FIG. 1 the numerals 18, 20, and 22 indicate the paths of three beams which converge to a point at a central aperture of the mask 15, instead of at the screen 16.

With zero beam deflection, the centers of deflection 24, 25, 26 of the beams lie in plane PP perpendicular to the axis AA, which plane is approximately at the axial center of the deflection yoke 17. This plane is referred to as the plane of deflection of the tube at zero beam deflection.

When the beams are deflected away from the paths 18, 2t), 22 to, e.g., the beam paths 28, 29, 30, the beams would, in the absence of dynamic convergence forces being applied thereto, converge to a cross-over before they reach the screen 16. To prevent such a premature cross-over, the beams are spread apart by dynamic convergence forces established by the convergence means 14. For example, the dashed lines 28' and 30', by comparison with the beam paths 18 and 22, illustrate the spread that the beams from guns 11 and 13 are given by the convergence means 14 when they are deflected by the deflection means 17 to follow the paths 28 and 30.

For a deflected beam, the center of deflection is defined as the point of intersection of the undeflected beam path and the rearward projection of the beam path after the beam leaves the influence of the deflecting field. Thus, the deflection centers of the deflected electron beams following paths 28,29 and 30 are respectively indicated by the points 32, 33, and 34.

As noted in FIG. 1, when the electron beams are deflected from the center of the screen, and proper convergence beam-spread is provided, the centers of the deflection of the beams move both forward and outward to the new points 32, 33, 34, and the plane of deflection PP moves forward a corresponding distance. The center of deflection of each beam thus defines a locus of points along a path. The forward shift of the centers of deflection along the tube axis AA is caused by the deflection of the beams by the yoke 17 away from the central portion of the screen; the outward shift away from the axis AA is caused by the spreading of the beam paths by the dynamic convergence forces produced by the convergence means 14.

As a result of the forward axial shift of the centers of deflection, the landing spots of the electron beams are shifted radially outward on the screen 16. This radial shift will occur substantially equally for each spot of any trio of beam spots. Unless a correction is made for this forward shift of the deflection centers, radial misregister of the beam spots with the phosphor dots will result. As a result of the outward shift of the deflection centers of the beams, the trio of beam spots will be spread apart, or enlarged, at outer portions of the screen 16. Unless this outward shift is compensated for, the trio of beam spots will be larger than its associated trio of phosphor dots, and degrouping misregister will result. Both radial misregister and degrouping misregister are manifest in all directions from the center of the screen, and increase as a function of increasing distance from the center of the screen.

Another type of spot-dot misregister can occur because of the inherent characteristics of the dot-printing system itself. In laying down a mosaic dot screen by a direct photographic process, a separate light exposure is made for each of the dot arrays of different phosphors, i.e., red, green, and blue emitting phosphors. In making each exposure, a small area light source is positioned at a location which is related to the deflection center of the associated electron beam. The light source location is different for each of the different exposures so that the dots of one array are displaced from the dots of the other arrays. In the case of a tube having a delta array of three guns, the location of the light source for each exposure is displaced from the central axis A--A of the tube.

In printing phosphor dots displaced from the center of the screen in the same or opposite direction as the direction the light source is displaced from the central axis AA, the apparent spacing of the light source from the zuris is foreshortened, or effectively decreased. This foreshortening will produce nonequilateral dot trios when the screen support is other than parallel to the plane of deflection, e.g., concave. Such a shift of the phosphor dots causes a distortion of the dot trios, and in tubes which do not exhibit an equal foreshortening effect of the beam spots, a consequent spot-dot misregister known as foreshortening misregister is produced. Even if misregister is not encountered, the distorted shape of the dot trio due to foreshortening is nevertheless objectionable because of the lessened phosphor dot coverage of the screen support surface which necessarily results.

Because foreshortening increases with increasing distance from the center of the screen, and because it occurs unequally in different radial directions from the center of the screen (varying for a given dot or spot from a maximum to a minimum along a tangential arc of 90) the dot trio distortion is non-uniform both radially and tangentially from the geometric center of the screen. As will be apparent from a later discussion with reference to FIG. 3, shift of the dots due to foreshortening may be either toward or away from the trio center depending upon the location of the light source and the overall dot printing system used.

Still another type of spot-dot misregister can occur because of the astigmatism of the deflection field produced by the deflection means 17. Although anastigmatic (or at least less astigmatic) deflection yokes can theoretically be fabricated, such yokes are not commonly available because of the manufacturing complexity thereof. The effect of yoke astigmatism is to distort the beam spot trio out of equilateralism as the beams are deflected away from the center of the screen. Astigmatism in yokes of conventional design cause the spot trio to have a greater vertical than horizontal dimension when the beams are deflected horizontally away from the center of the screen, and to have a greater horizontal than vertical dimension when deflected vertically away from the center of the screen.

The misregister which results from astigmatically distorted beam spot trios, is compounded by prior art dotprinting systems designed to correct degrouping m-isregister. Such methods cause a slight distortion of the phosphor dot trios which is opposite to (out of phase with) the beam spot trio astigmatic distortion and which increases with increasing deflection angles.

According to prior art screen printing methods the light source for each phosphor-dot-printing photoexposure is located substantially at a position termed the color center of the beam, hereinafter more specifically termed the first order color center of the beam. The first order color center of a beam may be defined as the intersection of a line extending through the centers of a given phosphor dot and its associated mask aperture with the plane of deflection (hereinbefore defined) associated with the given phosphor dot.

In a cathode ray tube which has proper spot-dot register, the first order color center thus corresponds to the deflection center; and for a given array of the same coloremitting phosphor dots, the first order color centers conhigher order color center of the associated beam, i.e., second order, third order, etc. A second order color center may be defined as that position in a given plane of deflection to which a given elemental screen area is exposed through a mask aperture which is adjacent to the mask aperture through which the given elemental area is exposed to the first order color center. A third order color center is that position to which the given elemental screen area is exposed through an aperture which is two apertures removed from the aperture through which the first order color center is exposed.

There are a plurality of second order color centers for each first order color center, the number thereof depending upon the mosaic pattern of the mask and screen. In cathode ray tubes having an equilateral array of three electron guns, the shadow mask apertures are systematically hexagonally arrayed, i.e., each aperture has six surrounding or adjacent apertures. Thus there are six second order color centers for each first order color center for such a tube geometry.

FIG. 2 illustrates the relationship of the second order color centers with their associated first order color center. In FIG. 2 the central tube axis AA (FIG. 1) is perpendicular to the paper at the intersection of the mutually perpendicular axes XX and YY. The points 24, 25, and 26 represent the first order color centers for the three undeflected beams from the electron guns 11, 12, and 13. These points correspond respectively to the like numbered centers of deflection of the three beams in the plane PP of FIG. 1. The first order color centers 24, 25, and 26 are equilaterally arrayed and symmetrically disposed about the central axis AA. Each first order color center is spaced a distance 5 from the tube axis AA along an axis which is perpendicular to the axis AA and which is termed the S axis of the dot-printing pattern. Positive displacements along the S axis are measured from the tube axis AA in the direction of the first order color center.

In FIG. 2 points 60, 61, 62, 63, 64, and 65 represent the second order color centers of the first order color center 26. The other first order color centers 24 and 25 have a similar array or system of second order color centers (not shown). The second order color centers for each of the first order color centers are spaced equidistantly from their associated first order color center on six equally angularly spaced radii, one of which passes through the central axis A-A and coincides with the S axis of the associated first order color center.

The spacing of a second order color center from its first order color center in ter-ms of the distance s may be obtained from the formula where q is the mask to screen spacing, L is the spacing between the color center and the screen, a is the spacing between adjacent mask apertures, and s is the spacing of the first order color center from the central axis of the tube in the zero deflect-ion plane of deflection. The derivation of this formula is treated in detail in an article by H. B. Law entitled A Three-Gun Shadow-Mask Color Kinescope in the October 1951 issue of the Proceedings of The IRE. This formula defines the relationship of s, L, q, and a for a tube geometry including an equilateral array of three guns, a flat mask, and a flat mosaic screen having an equilateral array of tangent phosphor dots at the center of the screen. When the formula is solved for the location of a second order color center in terms of the definition of a second order color center, it states that in the zero deflection plane of deflection the second order color center is displaced a distance equal to 3s from its associated first order color center. This relationship is indicated by the FIG. 2 diagram.

FIG. 3 is a flattened-out section of FIG. 2 taken along line 3-3 thereof and containing the axis AA and the two first order color centers 24 and 26. FIG. 3 illustrates how a second order color center, positioned in accordance with that shown in FIG. 2, does in fact conform to the definition as set forth above. In FIG. 3 the screen surface 16, shadow mask 15, plane of deflection PP, and the central tube axis AA corresponding to the same elements illustrated in FIG. 1, are schematically shown. Points 24 and 26 are two first order color centers and correspond to those points illustrated in FIG. 2.

In FIG. 3 the dashed lines 66 and 67 represent light rays from the first order color centers 24 and 26 which pass through a given mask aperture 68 and print a pair of phosphor dots at points 69 and 70. The rays 66 and 67 follow paths which cross-over at the mask 15 and then (like the dot-exciting electron beams) diverge in the region between the mask and screen. Point 60 in FIG. 3 constitutes one of the second order color centers for the first order color center 26 and corresponds to the second order color center 60 shown in FIG. 2. This specific second order color center is that one which is displaced from its first order color center in the same direction as is the central tube axis AA. Since each of the second order color centers is positioned a distance 3s from its first order color center, the second order center 60 lies on the S axis on the opposite side of the axis A-A from the first order color center 26 and is spaced twice as far from the axis AA as is the first order color center 26.

In practicing the method of this invention each dotprinting exposure is preferably made from a specific secnd order color center which bears the same relation to its first order color center as the second order center 69 does to its first order center 26. Point 71 is the second order color center which bears this same relationship to the first order color center 24. The solid lines 72 and 73 represent light rays from the second order color centers 60 and 71 which pass through mask apertures 74 and 75 adjacent to the given mask aperture 68 to print the two phosphor dots at points 70 and 69. The rays 72 and 73 are convergent in the region between the mask and screen 16.

Even though the phosphor dots of a tube are printed from second order color centers, the electron beams in operation of the tube are projected along first order color center paths and thus appear to originate from the first order color centers of the respective beams. Thus in second order color center printing the light rays which print a given phosphor dot pass through a different mask aperture than does an electron beam when it excites that given dot. Moreover, the mask apertures through which light rays pass to print the different dots of the same trio are different from each other. For example, the phosphor dots at points 70 and 69 are printed by light rays from the second order color centers 69 and 71 which pass through mask apertures 74 and 75, whereas these same phosphor dots at points 70 and 69 are excited by two electron beams which both pass through the same mask aperture 68.

In tubes wherein the mosaic dot screen has been printed from second order color centers, a unique spot-dot rela tionship is produced at the screen periphery. Each one of the different (e.g. three) color-emitting phosphor dot arrays produced by such a dot printing exposure includes a row of dots on the +8 (FIG. 2) side of the screen 16 (FIG. 3) for which there are no associated electron beam spots. Conversely, on the S side of the screen a row of beam spots will be produced in operation of the tube for which there are no associated phosphor dots. This condition results because the dot printing light orginates from a second order color center while the spot forming beam originates from a first order color center. Thus, although the associated beam spot and phosphor dot arrays are similar to each other and have corresponding spot and dot elements, these arrays are shifted with respect to each other on the faceplate. Consequently, a given beam spot excites the phosphor dot adjacent to that dot which has a position in the dot array corresponding to the position which the beam spot has in the spot array. For this reason, on the +5 side of the screen the imperforate peripheral portion of the mask 15 shadows the row of phosphor dots from all of these electron beams, one of which would otherwise excite these dots.

Since the S axis of each of the three dot printing exposures is angularly displaced from the others in a 3-gun tube as shown, the condition of unexcited dots and nonexciting beam spots exists completely around the periphery of the tube.

One of the major differences in the effects of printing from the first order color centers 24 and 26 or from the second order color centers 71 and 60 is concerned with the effect which a variation of tube magnification has in the relative positioning of the phosphor dots at points 69 and 70. As used herein, magnification is the ratio of the spacing Lq between the plane of deflection and the shadow mask to the spacing q between the shadow mask and screen. Consider for example, that the mask to screen spacing in FIG. 3 is decreased to a value q by moving the screen 16 closer to the mask 15 so that it lies in the plane EE. This is effectively a condition which exists at the edge of the screen in preferred practice where the magnification is greater. When such is the case, the light rays 66 and 67 emanating from the first order color centers 24 and 26 print phosphor dots at points 76 and 77. Onthe other hand, the light rays '72 and 73 emanating from the second order color centers 60 and 71 print phosphor dots at points 78 and 79. From this it can be seen that a decrease in the mask to screen spacing of the tube results in the phosphor dots being printed closer together when exposure is made from the first order color centers, but farther apart when exposure is 'made from the second order color centers. This difference, of course, is manifest because the first order printing rays 66 and 67 are diverging as they approach the screen 16, whereas the second order printing rays 72 and 73 are converging as they approach the screen 16. As is explained more fully hereinafter, this opposite effect of increased magnification in the tube geometry may be used to achieve the improved degrouping and astigmatism misregister correction.

Point represents a third order color center for the first order color center 26. In the plane PP it is displace-d from its associated second order color center 60 the same distance 3s as the second order color center 60 is from its first order color center 26. The dotted line 81 indicates the path of light rays from the third order color center 86 which pass through mask aperture 82 to print the phosphor dot at point 76. The aperture 82 is adjacent to the aperture 74 through which exposure is made from the second order color center 60, and is one more aperture removed from the aperture 68 through which exposure is made from the first order color center 26.

As shown by the intersection of light path 81 with plane E-E, use of third order color centers for dot-printing effect an even greater degrouping of the phosphor dots of a trio for a given decrease of mask to screen spacing than does second order color center printing. Exposure from the third order color center 80 would print a phosphor dot at the point 83 rather than at the point 78 where a dot would be printed by exposure from the second order c-olor center 60.

The greater rate of degrouping which third, and even higher, order color center printing yields, may be desirable in the making of some tube types. However, in the printing of phosphor dot screens for a direct-view color cathode ray tube for home television use which has a maximum deflection angle of second order color center printing is preferred. Therefore, so as to present a less complex explanation, the remainder of this disclosure will describe the use of second order color centers as applied to the making of such a 90 tube. However, it will be appreciated that even higher order color centers 9 may be used, and tubes with larger maximum deflection angles may be made.

An appreciation of the advantages of printing phosphor dots from second order color centers and the way in which such advantages are obtained can best be had by first understanding the limitation-s of prior art first order color center dot-printing systems, and then comparing such systems with second order color center printing described herein.

FIGS. 4, 5, and 6 illustrate the distortions of spot and dot trios in tubes of the prior art. The distortion of the spot trios is that which results from use of deflection yokes of known design whose fields are inherently astigmati-c. The distortion of the dot trios is that which results because of inherent characteristics of prior art first order color center dot-printing. For the purpose of more clearly illustrating the distortions, they are exaggerated in degree. FIG. 4 illustrates the shape and size of the spot and dot trios at the center of the screen; FIG. 5 illustrates the shape and size of the spot and dot t-rios at the 12 oclock and 6 oclock screen positions; and FIG. 6 illustrates the shape and size of the spot and dot trios at the 9 oclock and 3 oclock screen positions. In FIGS. 4, 5, and 6 the phosphor dots are represented, respectively, by the larger circles 90, 91, and 92, and the beam spots are represented, respectively, by the smaller circles 93, 94, and 95.

At the center of the screen (FIG. 4), the trio of phosphor dots 90 and the trio of beam spots 93 are equilateral with the centers of the phosphor dots 90 and the centers of the beam spots 93 being substantially coincident. The triangle 96 illustrates both the shape of the trio of spots 93 and the trio of dots 90. At the 12 oclock and 6 oclock screen positions (FIG. 5), the trio of beam spots 94 is astigmatically distorted from equilateralism to have a greater horizontal than vertical dimension. Triangle '97 illustrates the shape of the trio of beam spots 94. At the 9 oclock and 3 oclock screen positions (FIG. 6), the trio of beam spots 95 is astigmatically distorted from equilateralism to have a greater vertical than horizontal dimension. The triangle 98 illustrates the shape of the trio of beam spots 95.

In order to correct for degrouping misregister, the prior art provided a variable mask to screen spacing which decreased from the center to the edge of the screen and used a light retracting device in the dot-printing exposures. The decreased spacing was designed to selectively group the beam spots at the edge of the screen to produce a spot trio substantially equal in size to the spot trio at the center of the screen. In order to prevent this decreased mask to screen spacing from producing excessive crowding of the phosphor dots at the edge of the screen, an appropriate light refracting means, which tended to degroup the dots, was used in the dot printing exposure. However, because of the inherent characteristics of such an approach to degrouping correction, the phosphor dot trios were distorted in shape in a phase opposite to that of the beam spot trios and were moreover made smaller than that which would produce optimum spot-dot register.

In FIG. 5 the trio of phosphor dots 91 at the 12 oclock and 6 oclock screen positions are shown distorted to a non-equilateral triangular array which is horizontally compressed and/ or vertically elongated. The triangle 99 illustrates the shape of the trio of phosphor dots 91. The distortion of the dot triangle 99 is substantially opposite (out of phase with) that of the spot triangle 97, and the beam spots are so eccentric relative to the phosphor dots that they barely land thereon.

Also, both the phosphor dots 91 and the trio thereof are smaller than the corresponding dots 90 and trio thereof at the center of the screen. Because of the distorted shape of the dot trios at 12 oclock and 6 oclock the dots 91 have to be made smaller to prevent the upper two dots, as shown in FIG. 5, from overlapping each another. Although 'greater refraction of the dot-printing light rays could have been used to further degroup the dots 91 and thus permit the use of both larger dots 91 and a larger trio thereof, to do so would have even further aggravated the undesirable distortion of the dot trio shape and its misregister with the beam spots to the point that the beam spots might not fall entirely on their associated phosphor dots. Thus, a compromise between trio size and trio shape was made. Some degree of undesirability of each of these parameters was accepted in making the compromise.

In FIG. 6 the trio of phosphor dots 92 at the 9 oclock and 3 oclock screen positions are shown distorted to a non-equilateral triangular array which is vertically compressed and/ or horizontally elongated. The triangle illustrates the shape of the trio of phosphor dots 92. The resulting condition is similar to that described above for the 12 oclock and 6 oclock screen positions. The distortion of the dot triangle 100 is substantially opposite that of the spot triangle 98, and the dots 92 and the trio thereof are smaller than those at the center of the screen. As a consequence, the beam spots are so eccentric relative to the phosphor dots that they barely land thereon.

FIGS. 7, 8, and 9 illustrate the unique shaping of the phosphor dot trios and the relationship of the beam spot trios therewith at various representative screen positions in a screen embodying the invention. In FIGS. 7, 8, and 9 the phosphor dots are represented by the large circles 101, 102, and 103 respectively, and the beam spots are represented by the hatched small circles 93, 94, and 95, respectively. The beam spot trios in FIGS. 7, 8, and 9 are respectively, substantially identical with those of FIGS. 4, 5, and 6 since their astigmatic distortion occurs not because of tube structure or method of fabrication, but because of the nonuniformities of the deflection yoke which may be substantially the same in both cases.

The triangles 96, 97, and 98 which define the spot trios, also substantially define their associated dot trios. The desired nonequilateral shaping of the dot trios at the representative screen posit-ions shown in FIGS. 8 and 9, are opposite to, or out-of-phase with, the undesired astigmatic distortion of the prior art dot trios at corresponding screen positions shown in FIGS. 5 and 6. Because of this reversal of dot trio shaping, the phosphor dots according to this invention at any of the representative screen positions are more nearly concentric with their associated trio of beam spots.

Even though the phosphor dots 102 and 103 are smaller at the edge of the screen than are the phosphor dots 101 at the center of the screen, the trios thereof are not also undesirably reduced in size as was the case of the prior art dot trios shown in FIGS. 5 and 6. The phosphor dots 102 and 103 are made smaller than the phosphor dots 101 in order to permit the printing of an enlarged dot trio toward the edge of the screen without, at the same time, having the dots of one trio overlap the dots of a neighboring trio. Such an enlarged dot trio will more nearly match the size of the enlarged or degrouped spot trios. In the prior art the dots were made smaller in order to prevent their overlapping other dots of the same trio which could not be further enlarged to match the size of the spot trio without incurring an attendant, undesirable distortion of the dot trio.

To photographically print the phosphor dot trios as shown in FIGS. 7, 8, and 9, a suitable tube geometry must first be established. The tube geometry as discussed with reference to FIGS. 2 and 3 and the equation La q 38 determines the location of the preferred second order color center.

The specific points at which the phosphor dots will be printed at a given screen position is also dependent upon the magnification of the cathode ray tube at that screen position. Thus, in accordance with teachings hereinbefore set forth, the magnification of the tube is made to vary from a minimum at the center of the screen to a maximum at the edge of the screen. Such variation in magnification is preferably achieved at least in part by a mask-to-screen spacing which decreases from the center to the edge of the screen. The variation in magnification is such that when phosphor dots are printed from second order color centers, the size of the dot trios at a given screen position is increased while at the same time the associated beam spot trios produced in the subsequent operation of the tube are decreased. Considering separately dot trio shape distortion due to foreshortening, this appropriate selection of mask-screen spacing in itself substantially corrects for the differences in spot trio and dot trio size which causes degrouping misregister. Thus in correcting for degrouping misregister in second order color center printing, refractive devices in the path of the dot printing light rays as employed by the prior art may be omitted. Because refractive means is not used to effect a degrouping of the phosphor dots of the dot trios, misregister due to astigmatism of the beam spot trios is reduced. This advantage follows from the fact that the use of refractive means having special properties to effect dot degrouping in first order color center printing inherently distorts the dot trios in such a way as to aggravate the misregister resulting from spot trio astigmatism. Such distortion is reduced or avoided by using second or higher order color center printing.

In second order color center printing, refractive means is used for the correction of radial and foreshortening misregister, but not for degrouping misre-gister as in first order color center printing. Although refractive means in the path of the dot printing light rays may be used in higher order color center printing, it is used for a different purpose, in a different way, and produces a different effect than that of the prior art. Foreshortening of the spacing of the light source from the central axis of the tube occurs in second order color center printing as well as in first order color center printing. However, due to the nature of second order col-r center printing, the effects of foreshortening can be opposite from the effects encountered in first order color center printingnamely, in second order color center printing, the phosphor dots are shifted away from, instead of toward, the centers of their trios. Thus, whereas the occurrence and correction of foreshortening undesirably aggravated astigmatic misregister in first order col-or center printing, they advantageously alleviate it in second order color center printing.

Foreshortening results in some of the phosphor dots of a given exposure being shifted parallel to the S axis of that exposure. The amount of shift, which is different for different dots, increases with increasing distance from the center of the screen and varies from a maximum to zero through a 90 tangential are from the S axis (FIG. 2) to the PS (perpendicular to S) axis (FIG. 11). Thus, refractive means which will tend to compensate for fores'hortening should have refractive properties of a wedge or wedge-like element. As used herein wedge means a plate-like element whose thickness varies from a maximum to a minimum in one direction across its surface and is of relatively uniform thickness in the perpendicular direction. However, this is not to preclude some thickness variation or curvature being present in the perpendicular direction which might inherently result because of the particular wedge fabricating procedure used or might be provided by design to compensate for some secondary factor. Such wedge refractive properties may be: (a) provided in the form of a separate contoured wedge; (b) integrated with the refractive properties for correcting radial misregister in a single refractive element; or (c) provided as a combination of both (a) and (b).

In second order color center printing the spacing between the center of a phosphor dot and the center of its trio is determined by two substantially independently variable parameters: (1) the tube magnification, i.e., ratio of (Lq)/q and (2) the manifestation of foreshortening and the correction made there-for. A change of magnification simultaneously effects a substantially equal shift (but in different directions) of each of the dots of a trio and is effected substantially equally for dots on both the S and PS axes. Foreshortening and correction therefor effects a change of dot spacing independently for the three dots of a trio and is maximum for dots on the S axis and varies through a tangential arc to a minimum for dots on the PS axis. Each of the dots of the trio can be shifted differently from the shift given the other dots of the trio by using foreshorteningcorrection refracting means of different strengths for the three different dot printing exposures. Since these two dot spacing parameters are substantially independent of each other, the former can be used primarily to obtain a desired trio size while the latter can be used to obtain a desired trio shape. Such a freedom in dot trio sizing and shaping is not possible in prior art first order color center printing because the size of the dot trio is primarily adjusted by the refractive element that also sets the shape of the trio in correcting for foreshortening errors.

Apparatus 110, such as illustrated in FIG. 10, can be used in printing the phosphor dots. In the art of photographically printing mosaic dot screens, such an apparatus is referred to as a lighthouse.

The lighthouse comprises an open-top cabinet 111 having a shoulder 112 on which a bowl-shaped faceplate panel 114 of a cathode ray tube may be disposed. The faceplate panel 114 is adapted to be subsequently sealed at its open end 115 to another member (not shown) to form a completed cathode ray tube bulb. The panel 114 includes a surface 116 for supporting the phosphor screen 16 (FIG. 1) and a plurality of studs 118 on which the apertured shadow mask 15 (FIG. 1) can be removably mounted. Prior to mounting the panel 114 on the cabinet 111, the surface 116 is coated with a conventional photoresist material. On the external surface of the panel 114 are a plurality of bosses 119 which cooperate with mating recesses in the cabinet 111, whereby the faceplate panel 114 may be positioned in a prescribed orientation on the lighthouse 110.

At the base of the cabinet 111 is a housing 120 which contains a lamp 121 and a. tapered light conductor or collimator 122. The lamp 121 may, for example, comprise an ultraviolet emitting device such as a General Electric Company 1 kilowatt, high pressure, mercury arc lamp, type EH6. The collimator 122 is positioned above the UV lamp 121 and tapers away therefrom to a small area point 124. The point 124 is positioned in a selected plane of deflection P'P' at a predetermined distance from the central axis A-A of the faceplate panel 114. The lateral displacement of the small area light source 124 is preferably a distance of approximately 28 from the axis AA, substantially at a second order color center of the cathode ray tube system into which the mosaic dot screen being printed is to be incorporated.

The light housing 120 may, as shown, be mounted on a rotatable table 125 which can be indexed at a plurality of predetermined positions. Means 126 including a spring loaded plunger is provided which cooperates with mating depressions in the rim of the table 125 to fix the table 125 in these predetermined positions. Such indexing of the table 125 and housing 120 is designed to selectively position the light source 124 at different locations corresponding to, e.g., a selected second order color center for each of the plurality of dot printing exposures required.

A support bracket 127 disposed between the light source 124 and the faceplate panel 114 is supported on a plurality of legs 128 from the table 125. The bracket 127 has an opening 129 therein opposite the light source 124 across which suitable light ray refracting means 130 is supported. The refractive means 130 is thus maintained in a fixed 13 angular relationship with the light source 124 when it is moved.

Alternatively, the table 125 may be omitted, the light housing 120 mounted directly on the base of the cabinet 111, and the bracket 127 mounted directly on the wall of the cabinet 111. In such case the light source 124 and the refractive means 130 are fixed in their location. A plurality of similar lighthouses 110 are then provided for making the plurality of required dot printing exposures on a given faceplate panel 114. In each of such a plurality of lighthouses 110, the light source 124 is positioned with respect to a different one of the required second order color centers.

The refracting means 130 may, as hereinbefore stated, alternatively comprise: (1) a single element which is so contoured as to possess all the desired misregister-correcting refractive properties, (2) two separate elements, one of which possesses radial misregister correcting properties and one of which possesses foreshortening misregister properties, or (3) a plurality of elements which combine the features of alternatives (1) and (2). In the embodiment shown, the refracting means 130 is of the nature of alternative (3). The refracting means 130 comprises a plurality of separate elements 131 and 132 which are disposed backto-back either spaced from or contacting each other. The upper element 131 is fabricated according to US. Patent 2,885,935 Epstein et al. to provide both the desired correction for radial misregister and some wedge refraction properties for at least partial correction of foreshortening effects. The lower element 132, which may be cemented to the upper element 131, comprises a wedge contoured to provide the remaining correction for foreshortening misregister and to desirably shape the phosphor dot trios. In the combination optics 131-132, whatever the refractive properties of one of the elements, the other element is so contoured to complement the first as to provide the overall refraction desired. The contoured wedge 132 may, for example, be circular, be of uniform thickness in one direction across its surface (e.g., in the direction perpendicular to the paper), and vary in thickness in the perpendicular surface direction. Preferably, the contoured surface 134 tapers from both a thick side 135 and a thin side 136 of the wedge toward the center 137 where the surface 134 is substantially parallel to the planar wedge surface 138. The slope of the surface 134 is of the same polarity at all points from one end to the other and decreases from a maximum on one side of the center to a minimum (e.g. Zero) at the center and then increases to a maximum on the other side of the center. Such a contour results in substantially zero refraction in the S direction of light rays passing through the central portion 137 which print phosphor dots along the PS axis and in increasing amounts of refraction of light rays passing through the wedge at increasing distances toward either the thick side 135 or the thin side 136 which print dots displaced from the PS axis. The refraction of the dot printing light rays is manifest primarily in the direction of the S axis, and the wedge 132 (as well as the upper element 131) is so oriented that the refraction due to the wedge properties thereof is such as to shift the apparent source of the light rays in the -S direction. The specific dimensional shaping of the contoured surfaces of the two elements 131 and 132 are such that light rays passing therethrough will be refracted and print an array of phosphor dots so as to compensate for the foreshortening of the dots from the centers of their trios to a degree that will also alleviate astigmatic misregister.

Various design parameters of the cathode ray tube and the system in which it is to be used are first established before preparing the wedge 132. Among such parameters are the following: (a) the size and shape of the cathode ray tube bulb; (b) the electron gun structure of the tube and its position within the bulb relative to the phosphor screen; (c) the deflection yoke system used to scan the beams over the phosphor screen; (d) the dynamic con- 1 vergence system used to maintain the electron beams converged during scanning thereof; (e) the spacing between apertures of the apertured shadow mask of the tube; (f) the mask-to-screen spacing at the center of the screen, the size of the apertures of the apertured shadow mask, and the photographic dot printing parameters designed to produce tangent phosphor dots in the center of the screen.

The specific procedure to be used in practicing the method of this invention is somewhat dependent upon the particular design of the tube in question, the overall system in which it is to be incorporated, the design facilities available, etc. One procedure may be as follows:

(1) Design and fabricate the light refracting element 131 (FIG. 10) for correcting radial misregister for the tube in question in known fashion. The design of the refracting device may, for example, be in accordance with the teachings of the Epstein et al. Patent 2,885,935 as applied to the making of the 70 2lFBP22 direct view color cathode ray tube. Such design may be obtained mathematically by calculating the contour of the refracting element as based on the calculated or measured axial movement of the deflection centers of the electron beams as a function of the scanning deflection of the beams. Alternatively, such design may be obtained empirically by printing selected ones or all of the phosphor dots of a mosaic dot screen, fabricating this screen into a completed tube, operating the tube, and noting and measuring the radial misregister of the beam spot with its associated phosphor dot. From this measured misregister, the contour of the element 131 necessary to provide the desired correction may be calculated.

(2) Fabricate an apertured shadow mask according to an assumed mask contour. In order to simplify subsequent calculations, it may be preferable that the assumed mask contour be such as to provide constant magnification.

(3) Using the radial correction device 131 of step (1) and the assumed contour shadow mask of step 2), print a complete mosaic dot phosphor screen with the dot printing light sources located at first order color centers. For this particular step, first order color centers are used because uncorrected foreshortening misregister is so pronounced at this point in second order color center printing, that if the second order centers were used, subsequent evaluation of the dot arrays would be diflicult. In making the mosaic dot screen of this step, consideration should be given to mechanical errors, e.g., that caused by faceplate sag during evacuation of the tube, and to magnetic errors, e.g., that caused by the earths magnetic field.

(4) Fabricate a completed tube using the mosaic dot screen of step (3) and the assumed contour mask of step (2). Referring to FIG. 11, operate the tube and note and measure the degrouping misregister Ad on each end of the PS axes at various radial distances r r r etc. from the center of the screen for each of the different color phosphor arrays of dots. The measure of misregister Ad is the distance between the center of a beam spot and the center of its associated phosphor dot in the direction toward or away from the center of the phosphor dot trio.

As shown in FIG. 11, in a three-gun tube having three arrays of different color phosphor dots, six measurements would be taken at each radial distance. In FIG. 11 trios of red, green, and blue dots 150, 151, and 152 respectively, are shown at various radial distances r 1' and r on each of the red, green, and blue PS axes. Red, green and blue misregisters, Adr, Adg, and Adb respectively, are representatively shown at three screen positions 153, 154, 155, wherein the red, green, and blue beam spots 156, 157, and 158 are shown eccentric with their associated phosphor dots. The three values of Ad shown, together with the other three (not shown) at screen positions 160, 161, 162 are averaged to obtain the average misregister Ad at radial distance r Averaging misregisters at given radial distances simplifies fabrication of the calculated mask contour, since the mask can be made axially symmetrical.

Using the averaged misregister measurements of step (4), calculate the required value of q at each of the radial distances where misregister measurements are taken to give a revised mask contour which will eliminate the degrouping misregister noted in step (4). In making this calculation the following formula is used:

where P and represents the distance from the center of a phosphor dot to the center of its dot trio, p =Lq Ad is the average misregister of step (4), q is the revised mask-toscreen spacing and q is the assumed mask-to-screen spacing of step (2). v

(6) Fabricate a shadow mask according to the q values calculated in step (5).

(7) Using the mask of revised contour of step (6) and the radial correction element 131 of step (1), print a mosaic dot screen from second order color centers. Fabricate a complete tube using the screen of this step and revised mask of step (6).

(8) Operate the tube of step (7) and note and measure the spot-dot misregister along each of the S axes at various radial distances from the center of the screen for each of the different color dot arrays. This spot-dot misregister represents foreshortening misregister in excess of that which is corrected by the wedge properties included in the radial correction element 131 of step (1). These foreshortening misregister measurements are taken in a manner similar to those of step (4) described with reference to FIG. 11, except that they are taken on the three S axes instead of the three PS axes. These measurements may be averaged as in step (4) to design a refractive device which will provide an average correction for the excess foreshortening misregister for all three of the dot-printing exposures. Alternatively, the misregisters measured at the various radial distances along each of the three S axes may be considered as separate groups of data, and individual refractive correcting devices separately designed for each of the three color exposures.

(9) Determine the surface contour of the wedge-like refracting element 132 (FIG. 10) which will produce the required shift of location of apparent light source to correct the excess foreshortening misregisters noted in step (8). In making this determination, steps (10) through 16) as follows may be'used:

(10) Select a suitable material of known index of refraction and assume a thickness of a lano-parallel plate thereof. The assumed thickness should be such as to facilitate fabrication and handling of the refractive device and to provide a desired mechanical strength.

(11) Calculate the effects of refraction at various angles of incidence in terms of axial shift 5Z of the light source from an apparent plane of deflection P-P to an actual plane of deflection PP'. Use the following formula:

cos d) 527 Q] where t equals the assumed thickness of the refractive plate; N equals the index of refraction of the refractive plate; 95 is the angle of incidence corresponding to selected deflection angles; and 5Z is the apparent shift of the light source parallel to the axis AA. For convenience the calculated values of BZ may be plotted to construct a graph illustrating the non-linear variation of 6Z as a function of In Equation 2 62 represents a variable radial misregister error introduced by the presence of the planeparallel plate itself. If desired, correction for this radial error can be compensated for in the radial correction element 131 of step (1). However, as a practical matter this need not be done, but instead the source may be shifted to a compromise position by an amount AZ.

(12) Referring to FIG. 12, shift the axial position of the light source a distance AZ to a new plane PP' (FIG. 10) in the dot printing apparatus, where AZ is a selected fraction of the maximum 6Z distance from its apparent initial location so as to more equally distribute from the center to the edge of the screen the radial misregister error introduced by the refractive plate from which the foreshortening correction wedge is to be fabricated. In a cathode ray tube having a maximum deflection angle of a judicious distribution of the error is provided when zero error is established at a deflection angle of about 76.

(13) Dissassemble the tube from which the foreshortening misregisters were noted in step (8), and reposition the screen-hearing-faceplate onto the apparatus (now containing the plano-parallel plate and the element 131) which was used to print the mosaic dot screen in step (7). Referring to FIG. 13, shift the light source of the apparatus a distance AS laterally from its initial point 1'70 along one of the three S axes to a first point 171 in plane P'P' so as to obtain a register of the dot-printing light spot on the screen with the misregistered location of the beam spot that was noted in step 8) on that axis. Note and measure the amount of lateral shift AS; of the light source which was required to produce this register. Now shift the light source axially a distance AP and laterally a distance A8 to a second point 172 at which register of the dot-printing light spot on the screen with the misregistered location of the beam spot is again obtained. Note and measure the values of AP and A5 Repeat the step (13) procedure described above for a number of different screen positions to obtain a series of A8 AP, and AS values.

(14) Again referring to FIG. 13, calculate the slope of the contoured surface 177 of the wedge 132 (FIG. 10) at various angles of deflection to give the A8 values determined in step (13). Deflection angles on the -S side of the axis AA are given by the formula:

sin 0;sin 0 and 0 is the angle of incidence of the path 174 which a light ray must follow in entering a plano-p arallel plate in order to emerge from the plate to follow the desired dotprinting path 175; 0 is the angle of incidence of the path 176 which a light ray emanating from the intended light source location must follow in entering a refractive plate contoured as at 177 in order to emerge from the plate to follow the desired path A8 is the distance in the plane P'P' which the light source must be shifted fnom its actual location to obtain the desired register when exposing through a flat refnactive platethis distance is the apparent shift of the light source which the contoured wedge is to produce; A8 and AP are respectively the lateral and axial shifts from first to second light source locations which produce the desired register when exposing through a plano-parallel plate; Z is the distance along the axis AA from the plane of deflec-tion P'P' to the near, contoured surface 134 of the Wedge plate; tan 1; is the slope of the contour surface of the wedge at the point of incidence of the light paths 174 and 176; and N is the index of refraction of the materi l from which the Wedge plate is made.

17' Deflection angles on the side of the axis A--A are given by the formula:

tan 7) 1/N sin 0 -005 0 (6) where AS tan 0 tan G and the symbols of the formula are the same as Formula 4.

(15) Still referring to FIG. 13, calculate the positions on the surface of the refractive plate at which the slope values of step (14) fall. Use the following formula:

H=tan 01Z+AS1 where H is the distance along the S axis measured from the center of the wedge.

(16) Using the slope values tan 1; of step (14) and the positions or displacements H of step (15) calculate a series of thickness values of the wedge. Then using these thickness values determine a smooth contour, such as by fitting a polynominal to the slope values by known techniques, and fabricate a wedge 132 according thereto suitable for combining with the radial co-nrectio-n device 131 of step (1) in the lighthouse 110.

(17) Using the revised contour mask of step (6), the radial correction device 131 of step (1), and the wedge 132 of step (16), fabricate tubes, printing the dots thereof from second order color centers.

(18) Operate the tubes of step (17) and calculate any adjustments of mask contour and wedge contour 134 which are necessary to give optimum screen fabrication results. One or more of steps (1) through (16) may be repeated, if desired, in making these adjustments.

In making mosaic dot screens by a direct photographic method, it is known to slightly offset the light source from the color center where it would otherwise be positioned in order to compensate for small secondary factors. Examples of such factors are: (a) faceplate sag due to evacuation of the tube (see step 3 above), (b) refractions of the dot-printing light rays due to a finite thickness of glass in the paths thereof (see steps 11 and 12 above), and (c) the beam deflection effects of the vertical component of the earths magnetic field. Therefore, when stated herein that the light source is positioned substantially at a color center (e.g., higher order or second order color center) it is intended to include those instances when the light source is actually displaced from the color center in question for the purpose of compensating for these or other such factors. To be positioned substantially at a second order color center is, in a broad sense, meant to describe a positioning which is made primarily with respect to the second order color center instead of e.g. the first order color center, and wherein compensation displacement for any such minor purpose is measured from the second order color center rather than from the first order color center.

One example of practicing this invention in accordance with the teachings hereinbefore set forth involves the following specific dimensions, spacings, etc.:

(A) Spacing of each electron beam from the central tube axis A-A in the plane of deflection at zero deflection-0.205 inch.

(B) Spacing of the plane of deflection at zero deflection from the shadow mask 15 along the axis AA 11.205 inches.

(C) Spacing of mask 15 from screen 16 along the beam paths where r is the radial displacement from the 18 center of the mask in inches and q is the mask-to-screen spacing in inches:

(D) Spacing of light source from central tube axis AA0.410 inch on opposite side of axis AA from the associated electron beam.

(E) Spacing of light source from mask 15 along the axis AAl 1.372 inches.

(F) Spacing of interface 138 (FIG. 10) between the two refracting elements 131 and 132 from the mask 15 along the axis AA6.546 inches.

(G) Radius of curvature of mosaic dot screen 16 26.330 inches.

(H) Diameter of aperture in shadow mask 150.0l2 inch at center of mask decreasing to 0.010 inch at 9.35 inches from the center of mask.

(I) Spacing between adjacent apertures of mask 15 0.028 inch.

(J) Contour data for the wedge refractive element 132 (FIG. 10) where: H is the displacement in inches of a section of the wedge from the center thereof, positive values of H being in one direction from the center and negative values being in the opposite direction, t is the thickness in mils of the section at H displacement, and tan 1; is the slope of the contoured surface 134 (FIG. 10) at H displacement:

H (Inches) t (Mils) Tan 1; H (Inches) t (Mils) Tan 1;

H (Inches) t (Mils) Tan 1, H (Inches) t (Mils) Tan 1 (K) Contour data for the wedged-radial correction element 131 (FIG. 10) Where H is the displacement in inches of a section of the element from the center thereof and t is the thickness in inches of the section at H displacement. This data may be expanded by fitting a polynomial thereto to generate additional 1 values.

ALONG s AXIS IN PLUS DIRECTION H t H 1; H t

ALONG S AXIS IN MINUS DIRECTION H t H t H t US AND MINUS DIRECTIONS H t H 1; H t

(L) Beam spot trio shapes at various screen positions indicative of the deflection field characteristics of the yoke '17 and dynamic convergence system 14 as represented by spacings in mils between the red and blue spots, the

dots by a direct photographic process on a screen support of a cathode ray tube having a mnlti-apertured shadow mask and means for projecting an electron beam along its first order color center path and through the apertures of said mask and onto said screen support, and wherein the magnification of the tube varies from a minimum at the center of the screen to a maximum at the edge thereof, said method comprising:

(a) depositing a photosensitive coating on said screen support,

(b) exposing said coating through the apertures of said mask to a small area light source positioned substantially at a second or higher order color center of said beam so that light rays from said source which print a given phosphor dot pass through a different aperture than does the electron beam which excites said given dot, and

(c) developing said exposed coating.

2. The method of making the phosphor screen of a cathode ray tube having a multi-apertured shadow mask, a mosaic phosphor screen of different phosphors, and means for projecting a plurality of electron beams along their respective first order color center paths and the apertures of said mask and onto said screen, and wherein the mask to screen spacing of said tube varies from a maximum at the center of said screen to a minimum at the edge thereof, said method comprising:

(a) depositing a photo-sensitive coating on a screen support,

(1)) exposing said coating through said mask apertures to a small area light source positioned substantially at a second orderco'lor center of one of said beams,

(c) developing sai-d exposed coating, and

(d) repeating steps (a), (b), and (c) with said light source positioned substantially at a second order color center of another of said beams.

3. The method of making the phosphor screen of a cathode ray tube having a multi-apertured shadow mask, a mosaic phosphor screen of different phosphors, and a plurality of electron guns for projecting a plurality of electron beam along their respective first order color center paths and onto said mask and screen and wherein the magnification of the tube varies from a minimum at the center of the screen to a maximum at the edge thereof, said method comprising:

(a) depositing a photosensitive coating on a screen (b) exposing said coating through said mask apertures to a small area light source positioned substantially at a second order color center of one of the beams,

2.1 which color center is on the opposite side of the central tube axis from its first order color center,

() developing said exposed coating, and

(d) repeating steps (a), (b), and (c) with said light source positioned successively at a second order color center of another of said beams.

4. The method of making the phosphor screen of a cathode ray tube having a multi-apertured shadow mask, a mosaic phosphor screen of different phosphors, and means for projecting a plurality of electron beams along their respective first order color center paths and onto said mask and screen, and wherein the magnification of said tube varies from a minimum at the center of said screen to a maximum at the edge of said screen, said method comprising:

(a) depositing a photosensitive coating on a screen support,

(b) exposing said coating through said mask apertures to a small area light source positioned substantially at a second order color center of one of said beams,

(0) developing said exposed coating, and

(d) repeating steps (a), (b), and (c) with said light source positioned successively at a second order color center of another of said beams.

5. The method of making the phosphor screen of a cathode ray tube having a multi-apert-ured shadow mask, a mosaic phosphor screen of different phosphors, and a plurality of electron guns for projecting a plurality of electron beams along their respective first order color center paths and onto said mask and screen, and wherein the mask to screen spacing of said tube varies from a maximum at the center of said screen to a minimum at the edge thereof, for providing improved correction for degrouping and astigmatic spot-dot misregister, said method comprising:

(a) depositing a photosensitive coating on a screen support,

(b) exposing said coating through said mask apertures to a small area light source positioned substantially at a second order color center to one of said beams, which color center is opposite the central tube axis from its first order color center,

(0) developing said exposed coating, and

(d) repeating steps (a), (b), and (c) with said light source positioned successively at second order color center of another of said beams.

6. The method of making a phosphor screen of a cathode ray tube having a m-ulti-ape-rtured shadow mask, a mosaic phosphor screen of different phosphors, and a plurality of electron guns for projecting a plurality of electron beams along their respective first order color center paths and onto said mask and screen, and wherein the mask to screen spacing of said tube varies from a maximum at the center of said screen to a minimum at the edge thereof, which provides improved correction for degrouping and astigmatic spot-dot misregister, said method comprising the steps of:

(a) depositing a photosensitive coating on a support for said screen,

(b) exposing said coating through said mask apertures to light rays emanating from a small area source of light positioned substantially at a second order color center of one of said beams, which color center is opposite the central tube axis from its first order color center,

(c) retracting said light rays between said light source and said mask so that the rays incident on any given elemental surface area of said support appear to originate from a second order color center of the first order color center from which said one of the electron beams appears to originate when it is incident on the same given elemental area,

( d) developing said exposed coating, and

(e) repeating steps (a), (b), (c) and (d) with said point light source positioned successively at a second order color center of another of said beams.

7. The method of making the phosphor screen of a cathode ray tube having a multi-apertu-red shadow mask, a mosaic phosphor screen comprising a multiplicity of similar arrays of phosphor dots, there being one of said arrays associated with each of said mask apertures, and a plurality of electron guns having given spacial relationships with said screen for projecting a plurality of electron beams along their respective first order color center paths and through the apertures of said mask and onto said screen, and wherein the magnification of the tube varies from a minimum at the center of the screen to a maximum at the edge thereof, said method comprising:

(a) depositing a photosensitive coating on a screen support,

(b) exposing said coating through said mask apertures to a small area light source located at a point having -a given spacial relationship with one of said electron guns,

(c) developing said exposed coating, and

(d) repeating steps (a), (b), and (c) with a light source located successively at points each having said given spacial relationship with another of said guns,

said points being so located that the paths of light rays which print the different phosphor dots of the same dot array are convergent in at least a portion of the region between said mask and said screen.

8. The method of making the phosphor screen of a cathode ray tube having a multi-apertured shadow mask, a mosaic phosphor screen comprising a multiplicity of like arrays of phosphor dots, there being one of said arrays associated with each of said mask apertures, and a plurality of electron guns having given spacial relationships with said screen for projecting a plurality of electron beams along their respective first order color center paths and through the apertures of said masks and onto said screen, and wherein the magnification of the tube varies irom a minimum at the center of the screen to a maximum at the edge thereof, said method comprising:

(a) depositing a photosensitive coating on a screen support,

(b) exposing said coating through said mask apertures to a small area light source located at a point having a given spacial relationship with one of said electron guns,

(c) developing said expose-d coating, and

(d) repeating steps (a), (b), and (c) with a light source located successively at points having said given spacial relationship with another of said guns,

said points :being so located that the light rays which print a given phosphor dot pass through a different mask aperture than does the electron beam which excites said given dot.

9. The method of making the phosphor screen of a cathode ray tube having a multi-apertured shadow mask, a mosaic phosphor screen comprising a multiplicity of like arrays of phosphor dots, there being one of said arrays associated with substantially each one of said mask apertures, and means for projecting a plurality of electron beams along their respective first order color center paths and through the apertures of said mask and onto said screen, and wherein the magnification of the tube varies from a minimum at the center of the screen to a maximum at the edge thereof, said method comprising:

(a) depositing a photosensitive coating on a screen support,

(b) exposing said coating through said mask apertures to a small area light source so located relative to the path of one of said electron beams that the light rays which print a given phosphor dot pass through 23 a dilferent mask aperture than does said one electron beam when it excites said given dot.

10. The method of making a phosphor screen of a cathode ray tube of the type having a multi-apertu-red mask electrode, a phosphor screen including a mosaic of elemental areas of difierent color-emitting phosphors, and a plurality of electron guns for projecting a plurality of electron beams along their respective first order color center paths and through said mask and onto said screen, and wherein the magnification of the tube varies from a minimum at the center of the screen to a maximum at the edge thereof, said method comprising providing the mosaic of elemental phosphor areas of said screen by a photographic process involving exposure of a photosensitive layer to a light source which is disposed substantially at a second or higher order col-or center so that light rays from said source which print a given phosphor area pass through a diiferent mask aperture than does the electron beam which excites said given area.

11. In the method of photo-graphically laying down a phosphor screen including a mosaic of elemental areas of different color-emitting phosphors in a cathode ray tube of the type having a multi-apertured mask electrode; a screen support on which said screen is disposed; and a delta array of three electron guns symmetrically disposed about a central tube axis for projecting each of three electron beams through a dynamic beam convergence field, along its first order color center path and through said mask electrode and onto said screen, and wherein the mask to screen spacing of said tube varies from a maximum at the center of said screen to .a minimum at the edge thereof, the step of exposing photosensitive coatings on said screen support to light passing through an optical correcting lens and through said mask electrode from light sources positioned substantially at second order col-or centers of said tube.

12. The method of making a phosphor screen of a cathode ray tube of the type having a central tube axis along which is disposed a .phosphor screen including a mosaic of elemental areas of different color-emitting phosphors on a screen support, a rnulti-aper-tured shadow mask, and a plural electron gun assembly for projecting a plurality of electron beams along their respective first order color center paths and through said mask and onto 24 said screen, and wherein the magnification of the tube varies from a at the center of the screen to a maximum at the edge thereof, said method comprising:

(a) depositing on said screen support the mosaic of elemental phosphor areas of said screen by a process including a photoex-posure step wherein a photosensitive layer is exposed to light sources disposed successively substantially at second order color centers which are on the opposite side of said axis from their respective first order color centers.

13. The method of making a phosphor screen of a cathode ray tube of the type having a multi-apertured shadow mask, a phosphor screen including a mosaic of elemental areas of different color-emitting phosphors, and three electron guns triang-ularly disposed about a central tube axis for projecting three electron beams through dynamic beam convergence fields along their respective first order color center paths and through said mask and onto said screen, and wherein the magnification of the tube varies from a minimum at the center of the screen to a maximum at the edge thereof, said method comprising:

(a) providing the mosaic of elemental phosphor areas of said screen by a photographic process wherein a photosensitive layer is exposed to a light source disposed substantially at a second order color center which is on the opposite side of said tube axis from its first order color center and developing said coating.

References Cited by the Examiner UNITED STATES PATENTS 2,942,129 6/1960 May 313 3,003,874 9/1961 Kaplan 96-35 3,070,441 12/ 1962 Schwartz 9635 3,080,231 3/ 1963 Perry et a1 96--36 3,109,116 10/1963 Epstein et al 313-85 NORMAN G. TORCHIN, Primary Examiner.

ARTHUR GAUSS, ALEXANDER D. RICCI,

Examiners.

S. C. HATMON, C. BOWERS, Assistant Examiners. 

1. THE METHOD OF LAYING DOWN A PATTERN OF PHOSPHOR DOTS BY A DIRECT PHOTOGRAPHIC PROCESS ON A SCREEN SUPPORT OF A CATHODE RAY TUBE HAVING A MULTI-APERTURED SHADOW MASK AND MEANS FOR PROJECTING AN ELECTRON BEAM ALONG ITS FIRST ORDER COLOR CENTER PATH AND THROUGH THE APERTURES OF SAID MASK AND ONTO SAID SCREEN SUPPORT, AND WHEREIN THE MAGNIFICATION OF THE TUBE VARIES FROM A MINIMUM AT THE CENTER OF THE SCREEN TO A MAXIMUM AT THE EDGE THEREOF, SAID METHOD COMPRISING: (A) DEPOSITING A PHOTOSENSITIVE COATING ON SAID SCREEN SUPPORT, (B) EXPOSING SAID COATING THROUGH THE APERTURES OF SAID MASK TO A SMALL AREA LIGHT SOURCE POSITIONED SUBSTANTIALLY AT A SECOND OR HIGHER ORDER COLOR CENTER OF SAID BEAM SO THAT LIGHT RAYS FROM SAID SOURCE WHICH PRINT A GIVEN PHOSPHOR DOT PASS THROUGH A DIFFERENT APERTURE THAN DOES THE ELECTRON BEAM WHICH EXCITES SAID GIVEN DOT, AND (C) DEVELOPING SAID EXPOSED COATING. 